340 Lingual Dentition and Anatomy of 



below, convex above, with a rounded slightly prominent ridge along the 

 back, on which the skin is smoother than on the balance of the body, and 

 where the tuberosities are much less conspicuous. The foot has no 

 proper locomotive disk, tliough the broadly reflected mantle edge has 

 usually been counted as a portion of the foot and has given rise to the 

 impression that tlie foot of Onchidium is divided into three longitudinal 

 bands, of which the central is a locomotive disk. The eye-peduncles are 

 surely retractile, being found completely inverted in all the specimens 

 examined. This confirms the recent observations of Dr. Stoliczka.* 



We found no jaw in the specimens. 



The lingual membrane is broad. The teeth are arranged en chevron. 

 They are crowded closely together, the individual teeth and separate rows 

 of teeth overlapping each other. The central tooth has somewhat the 

 outline of a truncated cone, narrow and squarely truncated above, grad- 

 ually widening and curving outward toward the base, which is much 

 roader tlian tlie top, and is incurved with acutely pointed coi-ners. The 

 top of the tooth projects beyond the reflected cutting edge, which is small 

 and tricuspid. The first lateral is about the same size as the central. Its 

 squarely truncated apex extends beyond the reflected cutting edge, which 

 is bicuspid, the outer cusp subobsolete, the inner much larger and extended 

 into a long, broad, squarely truncated point, reaching almost to the base 

 of the tooth. This last is hidden behind the central, is long and gradu- 

 ally attenuated to its blunt base. The second lateral is of same sliape as 

 the first, but one-half longer and larger, the third and fourth laterals also 

 increase in like proportion. The general direction of all the laterals is a 

 curve outward fi-om the central. There are no distinct marginal teeth. 



Fig. 5 (pi. xvi) gives a group of centrals and laterals 

 from two adjacent rows of teeth. Fig. 3 shows one central 

 with its adjacent two laterals more enlarged, and purposely 

 separated. Fig. 4 shows one lateral in profile. 



This lino;ual is instructive from showins; a combination of 

 the characters of the quadrate teeth of Ilelicinoe and the 

 aculeate teeth of VitrinincB, the last most evident in the pro- 

 file. In profile, however, the reflected cusp is not of the 

 sharp, thorn-like character of Vitrina, Zonites, etc. We 

 should rather consider the teeth as decidedly quadrate, the 

 base of attachment, or plate, being extended beyond the 

 top of the reflected cusp. 



* Many years ago the eye-peduncles of OncMcHum were described as retractile by 

 Forbes and Hanley, yet of late years most authors have treated them as contractile, as 

 in Veronicella. 



